The present invention relates to downhole fluid driven motors used in the oil and gas drilling industry. In particular, the present invention relates to an improved roller vane motor.
There are various types of fluid driven motors which are designed to be incorporated into a drill string and are used to power (supply torque to) drill bits and other downhole tools. One type is the roller vane motor. The roller vane motor will normally be positioned in the drill string above the tool to be driven. A fluid (e.g. water, drilling mud, etc.) is pumped through the roller vane motor causing the motor to generate torque. FIG. 1 shows a cross-sectional view of a typical prior art roller vane motor 60. The motor 60 will generally comprise a stator 61 and a rotor 63. Rotor 63 will further include a series of flutes 66 with pockets 67 formed between flutes 66 and rollers 65 positioned in pockets 67. A fluid supply valve 68 will be formed through the center of rotor 63. Fluid passages 69 will communicate between supply valve 68 and pockets 67. Stator 61 will include interior stator walls which have varying radii. Portions of the stator wall will have a short radius (short radius portion 64a) approximate to the radius of flutes 66, such that flutes 66 forms a seal as they pass the short radius. Other portions of the stator wall will have a long radius (long radius portion 64b) which allows roller 65 to travel partially out of pockets 67. The stator will also have fluid exhaust ports 62 proximate said short radius stator wall portions. These fluid exhaust ports 62 are typically oriented radially outward from rotor 63 as suggested in FIG. 1.
In operation, fluid will be pumped down the drill string and will enter fluid supply valve 68. Rollers 65 which are facing a long radius portion 64b of the stator wall will tend to be pushed outward by the flow of fluid. Viewing in particular the roller designated 65a, it can be seen how rollers 65 will form a seal between rotor 63 and the long radius portion 64b of the stator wall and prevent the flow of fluid in the gap between rotor 63 and the stator wall. Another seal is formed between the flutes 66 of rotor 63 and the short radius portion 64a of the stator. Following the flow arrows in FIG. 1, it can be seen how fluid to the right of roller 65a may escape through exhaust port 62 and thus will be at the lower pressures existing outside the stator. On the other hand, higher pressure fluid flowing out of supply valve 68 into pockets 67 will be contained between the seal formed by flute 66 and short radius portion 64a and the seal formed by roller 65a engaging both the long radius portion 64b and the flute 66. The net effect of this arrangement is the accumulation of high pressure on the left side of roller 65a and lower pressure on the right side. Therefore, roller 65a will tend to move right toward exhaust port 62 and impart torque to rotor 63. Roller 65a will continue to impart torque to rotor 63 until roller 65a comes into contact with short radius portion 64a of the stator and roller 65a is forced into pocket 67, allowing the high pressure fluid behind roller 65a to escape through exhaust port 62.
One disadvantage with this prior art roller vane motor is that it is highly vulnerable to particulate matter in the power fluid. Very close tolerances are required where seals are intended to be formed, e.g. between flutes 66 and short radius portion 64a or between fluid passages 69 and the fins on supply valve 68. Particulates in the driving fluid either tend to stall the motor and/or seriously abrade the sealing surfaces. Additionally, the fins on supply valve 68 provide a very inefficient mechanism for sealing fluid passages 69. When these fins attempt to seal fluid passages 69, they present a critical short leak path between high and low pressure regions. Another disadvantage is the abrupt or violent manner in which rollers 65 will be forced into pockets 67 as the rollers approach short radius portion 64a of the stator. This abrupt action causes severe wear on both the roller and stator wall, thereby significantly reducing the useful life of the motor. It is believed that the degree of force with which rollers 65 are shifted in and out of pockets 67 is increased by exhaust ports 62 being oriented at a high angle in relation to the tangent of the stator wall at the exhaust port. For example, FIG. 1 illustrates a dashed line 71 tangent to the stator wall approaching exhaust port 62. Dashed line 72 shows the orientation of the exhaust port 62. The angle xcex1 demonstrates the orientation of exhaust port 62 to tangent line 71 is approximately 90xc2x0. Additionally, when the roller vane motor is starting up from a stationary position, there is a tendency for the motor to xe2x80x9clock upxe2x80x9d because the rollers are forced into exhaust ports 62 and tend to remain there. This tendency to lock-up is also believed to be caused or aggravated by exhaust ports being oriented at high angles relative to the tangent of the stator wall.
The present invention provides a roller vane motor for use in downhole drilling operations or various other applications. The roller vane motor has a housing and a stator positioned on an inside surface of the housing. The stator has an internal wall with an inlet port and an outlet port formed therein and the portion of this internal wall at the outlet port tapers open at an angle of less than 45 degrees relative to a tangent of the internal wall. A rotor assembly is positioned within the stator and includes: i) a rotor shaft, ii) a plurality of flutes extending from the rotor shaft; and iii) cylindrical rollers positioned between said flutes.